Methane is believed to be produced by various thermal and biogenic processes responsible for converting organic matter to solid carbonaceous subterranean materials such as coals and shales. When methane is produced in this manner, the mutual attraction between the carbonaceous solid and the methane molecules frequently causes a large amount of methane to remain trapped in the solids along with water and lesser amounts of other gases which can include nitrogen, carbon dioxide, various light hydrocarbons, argon and oxygen. When the trapping solid is coal, the methane-containing gaseous mixture that can be obtained from the coal typically contains at least about 95 volume percent methane and is known as "coalbed methane." The worldwide reserves of coalbed methane are huge.
Coalbed methane has become a significant source of the methane distributed in natural gas. Typically, coalbed methane is recovered by drilling a wellbore into a subterranean coalbed having one or more methane-containing coal seams that form a coalbed. The pressure difference between the ambient coalbed pressure (the "reservoir pressure") and the wellbore provides a driving force for flowing coalbed methane into the wellbore. As the ambient coalbed pressure decreases, methane is desorbed from the coal. Unfortunately, this pressure reduction also reduces the driving force necessary to flow methane into the wellbore. Consequently, pressure depletion of coalbeds becomes less effective with time, and is generally believed capable of recovering only about 35 to 50% of the methane contained therein.
An improved method for producing coalbed methane is disclosed in U.S. Pat. No. 5,014,785 to Puri, et al. In this process, a methane-desorbing gas such as an inert gas is injected through an injection well into a solid carbonaceous subterranean formation such as a coalbed. At the same time, a methane-containing gas is recovered from a production well. The desorbing gas, preferably nitrogen, mitigates bed pressure depletion and is believed to desorb methane from the coalbed by decreasing the methane partial pressure within the bed. Recent tests confirm that this process yields increased coalbed methane production rates and suggest that the total amount of recoverable methane may be as high as 80% or more.
As will be demonstrated by an Example contained herein, long-term injection of an inert gas into a formation may result in the production of a methane-containing gas having an inert gas fraction that generally increases in volume percent with time. This result may be undesirable as it may be necessary to lessen the concentration of injected inert gas in the produced methane-containing mixture before the mixture can be transferred into a natural gas pipeline or otherwise utilized.
What is needed is an improved process for the recovery of methane from solid carbonaceous subterranean formations that can provide a methane-containing gas that contains as little of the injected inert gas as possible to mitigate the costs associated with removing the injected gas from the produced methane-containing gaseous mixture.